Thank all for your comments and ideas.  Came up with this after reading a post to codeplex on escaping 's.  Turns out the magic use of "'"s  is.

'[dtmDocDate] >= CONVERT(datetime,...

SET @whereClause = [dtmDocDate] >= DATEPART(dy, '2020-09-01')

AND

SET @whereClause = [dtmDocDate] >= DATEPART(dy, 2020-09-01)

both receive errors on execution

Also tries with 's on beg and end of statement as the whole statement...

Yes, most definitely.

Since my last post I tried:

SET @whereClause = '[dtmDocDate] >= DATEPART(dy, 2020-09-01)'

with the same result.

However, I think I have discovered the underlying issue.  The script of which this...